Recently, according to an increase in demand for higher integration degree of a semiconductor device, a multi-layer wiring technology has become of much note. In the multi-layer wiring structure, a bottleneck against high speed operation of elements is capacitance between wires. In order to decrease the capacitance between wires, it is necessary to reduce the dielectric constant (or relative dielectric constant) of an interlayer insulating film.
Conventionally, a silicon dioxide (SiO2) film is generally used. Recently, however, other insulating film materials having lower dielectric constant are energetically developed. The conventional silicon dioxide film is formed by adding oxygen O2 or nitrous oxide N2O as oxidizing agent to material gas such as SiH4, Si(OC2H5)4 and the like, and by using a plasma enhanced CVD method and the like. The dielectric constant of silicon dioxide obtained in this way is approximately 4.0. On the other hand, it is reported that a carbon containing silicon oxide (SiOCH) film, which is formed by using methyl silane based precursor (for example, trimethyl silane or tetramethyl silane) as material gas and which is formed by using a plasma enhanced CVD method, has the dielectric constant of 3 or lower. In the SiOCH film, CH3 group as an end group is introduced into —O—Si—O— network, thereby reducing the density to decrease the dielectric constant thereof. For example, see page 3, column 2 through page 4, column 4 of the specification of U.S. Pat. No. 6,159,871, and page 11, column 4 through page 16, column 14 of the specification of U.S. Pat. No. 6,054,379.
However, the CH3 groups contained in the SiOCH film are often destructed in the O2 ashing process when trenches and vias are formed in the film. Therefore, the SiOCH film is susceptible to deterioration of film quality such as film contraction, moisture absorption and the like. This is because, the CH3 group is located in the end portion of the —O—Si—O— network, and easily reacts with O ions and radicals in the ashing atmosphere.
At present, in developing next generation devices, an effort is energetically performed to further reduce the dielectric constant by making the SiOCH film porous. However, when the SiOCH film becomes porous, the above-mentioned ashing damage becomes prominent. Also, in the via-first method which is most typical among the dual-damascene processes for forming wiring structure, side walls of the through holes (vias) formed in the process of the method are exposed to the ashing processes twice when the vias are formed and when the trenches are formed. When the side walls of the vias are deteriorated due to the ashing damage and moisture absorption becomes large, deterioration of yield of vias becomes large.
Further, when the SiOCH film is patterned, an SiO2 film (CAP-SiO2 film) is formed on the SiOCH film as a mask for patterning. However, adhesion of the SiOCH film with the CAP-SiO2 film is lower than that of the conventional film such as an SiO2 film, an SiON film, an SiN film, an HSQ film and the like. This is because, the CH3 groups contained in the film is hydrophobic, and affinity thereof with the SiO2 film is relatively low.